1. Field of the Invention
The present invention relates to wireless communication for performing media access control on the basis of carrier sense and, more particularly, to a wireless communication apparatus and wireless communication method for selecting a communication channel to be used on the basis of a channel search result, in a wireless communication system in which a plurality of users share a plurality of channels.
2. Description of the Related Art
Media access control (MAC) is control for causing a plurality of communication apparatuses which perform communication while sharing the same medium to decide how to use the medium in transmitting communication data. Owing to media access control, even if two or more communication apparatuses transmit communication data by using the same medium at the same time, there is less chance of the occurrence of a phenomenon (so-called collision) in which a communication apparatus on the receiving side cannot separate communication data. Media access control also reduces the chance of the occurrence of a phenomenon in which, despite the presence of communication apparatuses having transmission requests, a medium is not used by any of the communication apparatuses.
In wireless communication, since it is difficult for a communication apparatus to monitor transmission data while transmitting the data, media access control which is not premised on collision detection is required. IEEE 802.11, which is a typical technical standard for wireless LANs (Local Area Networks), uses CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance).
According to CSMA/CA in IEEE 802.11, in the header of a MAC frame, a period (called a duration) until the end of a sequence comprising one or more frame exchanges following the MAC frame is set. In the duration, a communication apparatus which is irrelevant to the sequence and has no transmission right waits for transmission upon determining a virtual occupied status of the medium. This prevents the occurrence of collision. On the other hand, a communication apparatus which has a transmission right in this sequence recognizes that the medium is not used except for a period during which the medium is actually occupied.
IEEE 802.11 defines that the status of a medium is determined on the basis of a combination of virtual carrier sense on a MAC layer as in the former case and physical carrier sense on a physical layer as in the latter case, and media access control is performed on the basis of the determination.
Jpn. Pat. Appln. KOKAI Publication No. 2003-87856 discloses a method of realizing wireless base stations which can be commonly used in a plurality of wireless LAN schemes in a wireless communication system in which a plurality of wireless LAN schemes with different physical layers are present. More specifically, a wireless base station is made to alternately generate the first notification signal on the first physical layer and the second notification signal on the second physical layer and transmit them to wireless terminals, and the first and second physical layers are switched in synchronism with the first and second notification signals. The wireless terminal corresponding to the first physical layer is allowed access only for a predetermined period of time after the transmission time of the first notification signal, whereas the wireless terminal corresponding to the second physical layer is allowed access only for a predetermined period of time after the transmission time of the second notification signal.
IEEE 802.11 using CSMA/CA has made attempts to increase the communication speed mainly by changing the protocol on a physical layer. With regard to the 2.4 GHz band, there have been changes from IEEE 802.11 (established in 1997, communication speed=2 Mbps) to IEEE 802.11b (established in 1999, communication speed=11 Mbps), and further to IEEE 802.11g (established in 2003, communication speed=54 Mbps). With regard to the 5 GHZ band, only IEEE 802.11a (established in 1999, communication speed=54 Mbps) exists as standard specifications.
As an approach to increase communication speed, a method of extending the frequency bandwidth of a channel is available. In this method, a channel having a given bandwidth and another channel having a bandwidth which is wider than that of the above channel are present in a given frequency band.
In high-throughput communication, in order to secure an idle channel having a wide frequency band, a channel search must be performed according to a communication bandwidth. However, since the conventional channel search scheme is designed to perform the channel search by a given communication bandwidth. Therefore, a channel having a different communication bandwidth cannot be searched for by the conventional channel search scheme.
For example, according to the conventional channel search scheme, in a wireless communication system in which a plurality of users share a plurality of channels, an idle channel is searched for by always using a given communication bandwidth. In this channel search scheme, a terminal having transmission data sequentially checks whether each channel is idle, by always using the given communication bandwidth. As a result, if any idle channel is detected, the terminal transmits data on the detected idle channel after completion of all of the channel searches. More specifically, in a wireless LAN standard IEEE 802.11h, in order to dynamically select a channel, the channel search is performed by a given bandwidth before communication, and a channel to be used in communication is determined.
In the above-described conventional channel search scheme, an entire system always uses a given bandwidth without any change of the bandwidth used for searching for the channel. Hence, in the wireless communication system for performing communication using a plurality of communication bandwidths, in some cases, the communication bandwidth to be actually used for data transmission does not match the bandwidth to be used for the channel search. In such a case, in the conventional channel search scheme, the channel search cannot be effectively performed in correspondence with the plurality of communication bandwidths. Also, even upon performing the channel search, a desired communication bandwidth may not be secured.